Wave-powered desalination of water

ABSTRACT

There is taught a device for the reverse osmotic desalination of water wherein the required energy is derived from waves.

GENERAL

The work culminating in this invention was performed under 1977 SeaGrant No. 4-6-158-44120 and 1978-79 Sea Grant No. 4-7-158-44120, underwhich the Government retains certain rights therein.

This application is a continuation in part of application Ser. No.266,660 filed May 26, 1981, now U.S. Pat. No. 4,421,466 which is acontinuation in part of application Ser. No. 076,217 filed Sep. 17,1979, now abandoned.

BACKGROUND OF THE INVENTION

Extensive regions of the world, particularly the nations denominated theThird World, lie in the path of the trade winds and are surrounded byseawater. Many of these regions lack a source of potable water and couldbe rendered arable and productive if irrigation water could be madeavailable. Particular regions of note include the Horn of Africa, theCaribbean Islands and large coastal areas of Australia.

The desalination of seawater has hitherto largely depended on fossilfuels as the power source. The rising cost and increasing scarcity offossil fuels render them substantially unavailable to poorer countriesof the world. Yet the steady increase in the world population putsincreasing strain on the water supplies and critically increases theneed for a practical and cost effective means to desalinate seawater.

Desalination of seawater by water wave-powered reverse osmosis has notreceived serious attention in the past, possibly because efficient andcost effective means for providing seawater at the required pressure wasnot known. However, some attention has been given generally to theefficient extraction of power from waves. For example, Count inProceedings of the Royal Society, London A, 363 559-573 (1978); IEEESpectrum 42-49 Sept. (1979) and Evans in Journal of Fluid Mechanics 77Part 1, 1-25 (1976) showed that the efficiency of a wave energyextraction device comprising a mooring means and a wave follower dependson the so-called natural period of the device and the power-extractiondamping of the motion of the wave follower.

The natural period of the device, as the term is here used, pertains tothe natural period of oscillation of the follower which, for maximumefficiency ought to match the period of the waves when in use. Amongcontrollable variables, the natural period depends primarily on the massof the wave follower which is kept low so as to minimize inertia. Afollower is most efficient, as has been found, when it isnon-Archimedean, i.e. is pulled underwater at least in part thusdisplacing more weight of water than its own weight.

The extraction of power from the movement of the wave follower withrespect to its mooring naturally causes damping which affects theefficiency of the device. The extraction of power in such a way as tomaximize the power yield is an important object of this invention as itpertains to the pump.

As is apparent, too much damping or constraint on the movement of thewave follower can result in no movement at all and thus no extraction ofpower; the classical physicist would say that although force isprovided, the force does not act through a distance and hence no work isdone. At the other extreme, it is apparent from similar logic, that ifno constraint is applied, no power is extracted. Maximum power isextracted somewhere between these extremes.

SUMMARY OF THE INVENTION

This invention comprises a wave-powered pump which can be employed forthe desalination of seawater employing reverse osmosis. It iscontemplated to utilize self-contained low-cost apparatus of relativelylow first cost and maintenance, which has a simple design permittingservicing by people unversed in the technical arts and which is durableand capable of withstanding the destructive effects of storms, tides andother like disturbances.

It has been found that a piston pump can be so adapted, particularly asto bore, with respect to the effective waterline plane area of the wavefollower and the required pressure, to produce a device maximizing theefficiency of power extraction from waves as described above. Such awave powered pumping device comprises:

(a) a non-Archimedean wave follower connected in sequence from top tobottom with

(b) a piston pump having a cylinder within which is mounted areciprocatory piston;

(c) attachment means connecting one of the pair consisting of saidcylinder and said reciprocatory piston with said wave follower and theother of said pair with mooring means selected from anchors, alone or incombination with a reaction unit;

wherein the ratio of the wave follower effective waterline plane area tothe piston cross sectional area is within the limits of 1000 and 6500optimally about 2000.

As to the process of using the device, one takes into account the waveheight and the required pressure, normally about 800 psi, by setting theproduct of the effective waterline plane area and the average waveheight divided by the product of the piston area and the requiredpressure in consistent units at between 50 and 200 optimally about 90,the stated areas being in any convenient consistent units; the waveheight being in inches and the pressure in pounds per square inch.

Thus, having in hand the described wave powered pump capable ofproviding seawater at pressures of 800 psi or even more, combinationwith a reverse osmosis device, as is known, provides the means to causethe oceans to desalinate themselves. It is an object of this inventionto cause at least a modest part of said oceans, especially those in thetrade wind regions, to do so.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings constituting part of the specification arepartially schematic representations of preferred embodiments of thisinvention, in which:

FIG. 1 is a side elevation of an apparatus comprising desalination meansshown in installed position in a saltwater ocean or bay,

FIG. 2 is a plan view of the wave follower of the apparatus of FIG. 1,

FIG. 3 is a vertical cross-sectional view of the wave follower of FIG. 2taken on line 3--3,

FIG. 4 is a plan view of the reaction unit of the apparatus of FIG. 1,

FIG. 5 is a vertical cross-sectional view of the reaction unit of FIG. 4taken on line 5--5 comprising a reverse osmosis unit for desalination,

FIG. 6 is a view, partially in cross-section, of one embodiment ofwater-processing system which can be employed with the apparatus ofFIGS. 1-5,

FIG. 7 is a view, partially in cross-section, of an especially preferredembodiment of water-processing system suitable for use with theapparatus of FIG. 1-5, and

FIG. 8 is a cross-sectional view of a preferred type of check valvewhich can be used in the water-processing systems of FIGS. 6 and 7.

FIG. 9 is an elevation view of a desalination apparatus as installed.This apparatus is most preferred in regions of relatively small tidalrange.

FIG. 10 is an elevation view of a desalination apparatus as installed.This apparatus is preferred in regions where deep embedment anchors areimpractical.

FIG. 11 is a plan view showing the arrangmenent of some of thecomponents of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The experiment, to be described, which is illustrative of many carriedout in a wave tank and in the sea, shows the manner in which the ratiosof piston cross sectional area to wave follower waterline plane areawere arrived at so as to provide efficient piston pump coupling betweena non-Archimedean wave follower and a reaction unit, arrangedessentially as illustrated in FIG. 1.

The experiment, carried out in a 120'×8'×5' deep wave tank, employed a1' diameter polystyrene foam wave follower. A 16" diameter disk 8" thickhaving an immersed weight of 4 lb. was employed as reaction unit. Thespring constant was 3.6 lb./ft. The water depth was 35". The wave heightwas 2" and the wave period was 1.4 sec. The piston cross sectional areawas 0.1 in². The back pressure downstream from the pump was varied from25 psi to 60 psi.

Efficiency as a function of back pressure was calculated as the ratio ofthe product of the rate of pumped water in pounds of water per unittime, and pressure expressed as feet of water head divided by the energyof the intercepted wave front (1') in foot pounds. The energy of thewave front was calculated in the conventional manner, for example,according to the U.S. Army Coastal Engineering Research Center ShoreProtection Manual.

A plot of back pressure versus efficiency was bell shaped, convex upwardhaving a maximum of 20.8% efficiency at 25.6 psi.

The scale of the equipment of this experiment is 1/15th that of acontemplated trade wind device. The experiment shows that the pistonarea was about twice that which, in the experimental scale, would beoptimum; that an optimum piston area of about 0.05 in.² would, on 15fold scaleup, provide a pressure of 800 psi, as is needed for reverseosmosis desalination, and up to about 10,000 gallons of pressurizedwater per day under trade wind conditions (2-3 ft. waves of 3-10 secondperiod).

The ratio of the wave follower effective waterline plane area to thepiston cross sectional area was 2262, near the optimum value. Theeffective waterline plane area is taken to be the mean waterline planearea as the waterline moves up and down during a complete pump stroke.

Particular ratios are selected according to the expected wave height andthe required pressure. It will be useful to the designer to note thatthe product of the effective waterline plane area and the wave heightdivided by the product of the piston area and the required pressure, inconsistent units, will lie between 50 and 200, optimally at about 90.

The further description following gives details of devices suitable formeeting the potable water needs of numbers of people. The relativesimplicity of the devices and hence their cost effectiveness isapparent.

In FIGS. 1-5 the power supplier of this invention comprises the wavefollower denoted generally as 10. The shape of the wave follower ofFIGS. 9, also designated as 10, is preferred because the conicalunderside tends to make the follower ride more easily in a real sea,whereby to avoid pounding, for example.

In the example herein described, the wave follower 10 is pulled downinto the water approximately two feet. The objective is to have the wavefollower respond to the buoyant force exerted by a wave crest, as wellas the wave-induced hydrodynamic drag and inertia forces imposed on thewave follower's immersed portion. However, wave follower 10 should belight enough so that it can keep up with most waves.

Although our apparatus is not highly frequency-dependent, it isadvantageous that its natural period lie in the range of wave periods inwhich it will be operating. For the trade wind situation, the periodrange is from about 3 to 10 seconds.

A preferred design of wave follower 10 utilizes a 15' diameter×5' longvertical cylinder with cone weighing no more than about 8,000 lbs.Generally, the size of the wave follower will be determined by theaverage wave energy flux at the deployment site and the piston crosssectional area as described above. In practice mooring buoys might beused.

The design of wave follower of FIGS. 1-5 detailed comprises a mass ofpolyurethane foam 11 sandwiched between upper steel plate 12 and lowersteel plate 13 and held together by stay bolts 14 and axially disposedeye bolt 15.

Wave follower 10 is tethered to the remaining apparatus by sacrificiallink 20, which is designed to part under the stress of extreme storms,tidal waves or similar abnormal wave conditions, thereby permitting themajor part of the system to drop safety to the sea floor, from which itcan be easily recovered later, whereas the wave follower is set adriftas a highly visible floating object which can be recovered fortuitouslywherever found.

Description of the seawater pump assembly 21 and appurtenances isdeferred until later, so that the remainder of the wave dynamic systemcan be now detailed.

The non-buoyant hydrodynamic reaction unit 22 of FIGS. 1, 4, and 5 canbe a highly useful component of the invention having, as primaryfunctions, the capability of permitting operation in areas withsignificant tidal ranges and reduction of the mooring strains. It alsoacts as a buffer during storm conditions. The efficiency of devicesemploying reaction units is about the same as those employing simpleanchor mooring means.

Reaction unit 22 should preferably have an effective mass at leastequivalent to about 10 times the mass of the water displaced by the wavefollower.

As detailed in FIGS. 1, 4 and 5, reaction unit 22 can be constructed asa hollow steel casing comprising top and bottom plates 26 and 27,respectively, joined through welding by hexagonal shell 28. Typically,the immersed weight can be 4,000 to 12,000 lbs., diameter at least 20ft. and height 10 ft. provided that flow retarders 23 are incorporated.Weight distribution should, of course, be such that unit 22 alignsitself vertically with wave follower 10 when the latter is at rest. Flowretarders 23, which are simply upwardly deflected tabs opening intoshell 28, brake the upward movement of reaction unit 22, so that therise time of the reaction unit is longer than its fall time. Thisensures that the system remains under tension even when it is subjectedto high frequency waves. An alternative design for reaction unit 22 isto streamline it so as to decrease its fall time relative to its risetime. In both cases, the limit of fall of reaction unit 22 will bedetermined by the real mass, which is supported on elastomeric returnsprings 65 (FIG. 1).

Axial tie rod 30, provided with connection eyes 30a and 30b at top andbottom ends, locked to plates 26 and 27 by flanges 26a and 27a,respectively, and flexible tether 33 constitute the coupling meansbetween seawater pump assembly 21 and ballast block 31. A three-pointmooring 32 made up of steel chains 100 ft. long running to individualanchors, not shown, coming together at the ballast block 31 provides asatisfactory arrangement, with a single line 33 running to reaction unit22. In calm conditions, line 33 will be slack. Mooring configurationscan, of course, vary widely, depending on local bottom sediment and waveand current conditions.

In some circumstances a simpler reaction plate may be preferred such asthat designated 22 in FIGS. 10 and 11. In that embodiment the plate is asimple square ribbed steel plate having an area normally larger thanthat of the wave follower effective water line plane area, depending onits mass. In this connection, the plate could be ballasted. As shown inFIGS. 10 and 11, the plate is loosely tethered to four anchors 85 so asto hold the assembly generally in the same place on the sea bottom whenat rest but still leave the plate free to accommodate to upward forceswithout placing undue stress on the anchors.

Returning now to the seawater pump assembly 21 and appurtenances,reference is made especially to FIGS. 1, 5, 6, 7 and 8.

The single-acting pump, generally denoted at 38, FIGS. 1, 6 and 7 cancomprise the loose-fitting reciprocatory piston design which is thesubject of application Ser. No. 044,540, now U.S. Pat. No. 4,221,550.This pump embodies a polymeric ball 39 fabricated from Adiprene® orsimilar elastomeric material which is supported in a concavity machinedin the upper face of piston 40 and retained therein by compressionspring-biased lock member 41. On the piston upstroke, polymeric ball 39is compressed between the piston face and the lock member, and itsdiameter in a plane at right angles to the pump axis increases toeffectively seal against water leakage past piston assembly 39, 40, 41,thereby delivering seawater under high pressure out through deliveryline 42, thence, through check valve 43 to accumuator 44.

Conversely, on the piston 40 downstroke, check valve 43 closes and checkvalve 45, connected in series with intake line 46, opens, filling thepump with seawater for the next following pumping stroke.

Referring to FIG. 8, check valves 43 and 45 preferably havefrusto-conical seats on the valve closure ends accommodating freelyslidable mating frusto-conical valving members 43a and 45a,respectively. These are each provided with short foot members 43b and45b on their reverse sides, impinging on the annular seats machined onthe valve open ends of the valve housings.

In a simple piping circuit for desalination with particular applicationto the apparatus of FIG. 1 (shown in FIG. 6), raw seawater passes fromaccumulator 44 (not shown in FIG. 1) via pressure relief valve 50, whichproduces flow at pressures about 800 lbs./sq. in. via tubing 51 to thehigh pressure side of reverse osmosis module 52, which can be acommercially available unit such as that marketed under the trade name"Permasep"® by E. I. du Pont de Nemours & Company. Reverse osmosismodule 52 delivers fresh water via tube 54 to a storage tank or otherfacility not shown, whereas waste brine is exhausted to the surroundingwater via line 55.

Because reverse osmosis module 52 is the most expensive component of theapparatus, it is preferred to house it within reaction unit 22,conveniently installed within protective pipe well 58 supported ontransverse spider 58a. The position of the module is, of course, notcritical; it could be placed anywhere in the apparatus, on shore, onshipboard, or on the seafloor, for example.

As shown in FIG. 1 particularly, seawater pump 38 is mounted on a cradle59, the upper plate 60 of which is secured via yoke 61 to pump pistonrod 62. Lower plate 63 is attached to the pump cylinder 64 and toconnection eye 30a and a multiplicity of circumferentally spacedelastomeric springs 65 in tension, which can conveniently beseawater-resistant rubber shock cords, secure plates 60 and 63 together,thereby constituting external return springs impelling pump piston rod62 on its down or refill stroke in an essentially inertia-free manner.

The spring constants on elements 65 are preferably in the range of about500 to 1800 lbs./ft. depending on average wave conditions. Returnsprings 65 should be preloaded to afford support for the immersed weightof reaction unit 22 when pump piston rod 62 is about one third down inits stroke and to pull wave follower 10 into the water at least in part.

A sand well 70 is preferably utilized to prefilter the seawater beforeintake to pump 38. This sand well can comprise a polymeric tube with awell screen at the bottom, not shown, jetted into the sand to a depth of25 to 50 ft; depending on the ambient water quality and sandcomposition.

Referring to FIG. 7, there is shown an embodiment of a water-processingsystem according to this invention wherein seawater is desalinated,which incorporates all of the same components as the system of FIG. 6,denoted by the same reference numerals, except for the addition of ahighly advantageous energy recovery system.

Thus, instead of discharging waste brine from reverse osmosis module 52direct to the sea via exhaust line 55, the brine is returned to theunderside of piston 40 in proper timed sequence with the piston rod 62displacing the volume lost as fresh water through the reverse osmosismodule, so that its substantial energy, typically, above 700 lbs./in.²pressure, aids the pump delivery stroke effort contributed by drivecradle 59.

To accomplish this, waste brine line 55a is directed throughconventional fluidic switch 71 and thence via pump boost line 72 openinginto the piston rod end of pump 38. Switch 71 derives operative powerthrough signal line 73 connected in shunt from high pressure seawaterdelivery line 42. The time sequence of fluidic switch 71 is as follows:(1) high pressure during seawater delivery via line 42 is communicatedthrough line 73, thereby opening passage for exhaust seawater flow fromreverse osmosis module 52 through line 55a via switch 71 and pump boostline 72 to the rear side of piston 40; (2) upon completion of thepumping stroke, the pressure above piston 40 drops to ambient, as doesthat in signal line 73, thereby closing the fluidic switch seawatercommunication with pump 38, simultaneously opening discharge of thebackside of piston 40 to the sea via drain line 74.

Referring to FIG. 1, the calm water equilibrium position of wavefollower 10 is shown as stabilized at seawater level A, under which(relatively rare) conditions the apparatus will be non-operative.However, when wave action ensues, particularly when the trade windsblow, sustained (practically continuous) medium-to-heavy wave actionoccurs, as denoted by broken line B, and full pumping power isdeveloped.

In operation, the total depth at the site of deployment is preferably50-80 feet. Under these circumstances the line between wave follower 10and reaction unit 22 can be from about 10-50 feet, depending on thetotal depth of deployment.

For service in trade wind regions, pump 38 preferably has a pistondiameter in the range of about 3.8" to 5.5" and a total working borelength of about 60" in reaction unit-comprising assemblies. When directanchor mooring is employed, the bore length is made longer toaccommodate the tidal range. The disclosed dimensions correspond toratios of wave follower effective waterline plane area (dia. 15') topiston cross sectional area of about 2244 and 1071 respectively.

An alternative construction for reaction unit 22 is to employ amonolithic slab of concrete as a substitute for the hollow drumconstruction hereinbefore described. This alternative design permitsutilization of a much more compact form, particularly as regardsthickness. Reverse osmosis unit 52 can nere conveniently be accommodatedhorizontally within a protective recess provided in the upper surface ofthe slab.

The apparatus hereinbefore detailed is capable of producingapproximately 1500 gals./day of potable water from full strengthseawater at a continuous power consumption of approximately 1.5kilowatts. Thus, with a wind speed of 16 knots, a fully developed seawill have a 4.5 ft. significant wave height and a statistical meanperiod of 4.6 sec. Using the method for calculation of wave power givenby R. S. Arthur (Scripps Institute of Oceanography Wave Report #68,1947), it can be safely assumed that there exists an average wave energyflux of 1.4 kw. per foot of wave crest. Therefore, a desalination systemoperating at only 12% efficiency for wave energy extraction and at only60% efficiency for the system hydraulics, with a 15 foot wave frontage,should easily produce 1500 gallons/day of fresh water.

It should be mentioned that the system components, if constructed ofseawater-resistant materials, as described, utilizing a reverse osmosismodule of the reliability specified, should have a long, trouble-freelife in desalination service, excepting, of course, the relativelyinfrequent hazards of unusually turbulent seas, such as those occasionedby violent storms, extremely high tides and collisions with water craft.Even then, the design is such that sacrificial link 20 will part underpotentially destructive forces, saving the structure from ruin andmaking it possible to restore it to service with a minimum of effort andtime loss, providing that a low-cost reserve wave follower 10 can besubstituted as replacement (or the original wave follower 10 recapturedand returned to service).

If desired, it is practicable to provide anchoring lines running to theouter edge of the reaction unit if one is used. This prevents excessiverotation of the assembly responsive to changing wind action. With suchan anchoring system, apparatus recovery after loss of a wave follower 10is facilitated.

Finally, it will be understood that the intermittent rate of potablewater delivery resulting from varying wind strengths and wave actionshas no effect on overall operation, since potable water is stored intank facilities night and day on an average as-delivered basis, whichneed not inconvenience users in the slightest.

The foregoing description is directed to a preferred embodiment of thisinvention; however, it will be understood that relatively widevariations in design are feasible and, depending upon local conditions,even desirable.

Thus, it is practicable to utilize double-acting pumps as substitutesfor single-acting designs, provided that the design of associatedcomponents is modified accordingly.

Also, the invention can dispense with hydrodynamic reaction unit 22 perse if certain operating limitations are accepted, or if alternateapparatus is substituted for the reaction unit.

The primary purpose of reaction unit 22 is to accommodate operation totidal changes, storms and unusual oceanic turbulence generally. Totaltide swings in near equatorial regions usually range from about 0 to 4feet, under which conditions loose mooring with hydrodynamic reactionunits of the design described is entirely practicable in permitting wavefollower 10 to respond to wave movement essentially independent of tidallevel.

Another way of achieving accommodation to tidal changes and storms wouldbe to provide a spring-loaded anchor cable winch, securely anchored tothe seafloor, equipped with automatic controls effecting pay out orreel-in of the mooring connection to pump 38, so that there is alwaysmaintained an optimum slackness subject to demand.

Also, it is practicable to anchor wave pump 38 directly to the sea floorat deployment sites with relatively small tidal ranges, on the order of0 to 2 feet, dispensing altogether with any intermediate reaction unit.However, the pump stroke must be increased to accommodate the tidalrange; otherwise, there could be periods when the follower is entirelysubmerged in the water and, thus, severely curtailed in its response towave movement. Moreover, such an arrangement could subject the mooringto excessive physical stresses, depending on existing conditions.

We have found that the security of the design hereinbefore detailed isquite broadly independent of wave height greater than that required togive maximum performance and the usual periodic variations of tide,weather and the like.

Our studies have confirmed that wave follower 10 should benon-Archimedean. Such a wave follower, when pulled down into the waterappreciably (e.g., 12" to 24" for a follower 15' in diameter),effectively becomes part of the ambient ocean or bay, so that thefollower responds additionally to orbital movement of the waterparticles rather than to vertical displacement solely. Wave follower 10is preloaded by elastomeric return springs 65 which thereupon balancethe mass of reaction unit 22, preferably so that piston 40 is at about7/8 inspiration position. (This arrangement permits use of a reasonablelength, i.e., 60" pump, whereas, with direct sea floor mooring, a pumpwould have to be 60" long with the additional of about one foot in pumplength for every foot of tidal range if continuous pumping service isdesired.) Other essentially inertia free forces could be used, such asair springs, in place of elastomeric return springs.

In general, a wave follower 10 weighing less than 8,000 lbs, can beutilized with a minimum weight reaction unit 22 weighing about 4,000lbs. or a maximum weight unit weighing up to about 12,000 lbs.

Such a design functions well at wave periods in the relatively broadrange of 3 to about 10 seconds. Surprisingly, the difference inefficiency between a fixed moored design and the design hereinbeforedetailed is not more than ±10% absolute.

FIGS. 9, 10, and 11 describe somewhat simpler embodiments of theinvention which, for this reason, may be preferred over the devicesheretofore described. The same numbers appearing in different drawingsrepresent either the same components in the different drawings orcomponents having an analagous function, as will be apparent from thedrawings.

FIG. 9 is an elevation view of a desalination device suited for usewhere the tidal range is relatively narrow, for example in Puerto Rico.When so deployed the length of pump cylinder 64 and piston rod 62 areselected to accommodate the tidal range. Return springs 65 as connectedto piston rod 62 and pump cylinder 64 are selected according to thecriteria previously set out whereby wave follower 10 is renderednon-Archimedean. As is apparent, the springs, preferably rubber,function to fill the pump while wave follower 10 descends; the pumpstroke occurring on rise of wave follower 10. Force from wave follower10 is transmitted via tether 71 to piston rod 62 via sacrificial link20. The lower end of cylinder 64 is attached via tether 71 and optionalcoupling 72, which can be useful for reducing wear in the tether, to ananchor 73 or other mooring means. A junk automobile has been foundsatisfactory as a mooring means.

With fall of wave follower 10, water from sand well 70 is drawn viafirst check valve 74 into cylinder 64; with rise in wave follower 10,water in cylinder 64 is expelled via line 84 and second check valve 75into accumulator 44. Pressurized water leaves the bottom of accumulator44 and, via line 76, enters the shell side (not specifically shown) atthe lower end of reverse osmosis module 52. Branching from line 76 ispressure relief valve 50 which releases pressurized water to the seashould the pressure rise above a predetermined valve. Also branchingfrom line 76 is pressure gauge 77 which is useful in adjusting thedevice. Emerging from the tube side (not specifically shown) of reverseosmosis module 52 at the bottom is potable water line 54 which deliversthe desalinated water wherever desired, usually ashore. Connected to theshell side (brine) of reverse osmosis module 52 at the upper end ispressure relief valve 78, a pressure gauge 79 and a flow control valve80 which releases brine into the sea. As a rule, approximately 15% ofthe water taken into the device is recovered as potable water and 85% isreturned to the sea as somewhat concentrated brine. Theaccumulator-reverse osmosis module assembly is, in this embodiment, heldupright by float 81 in tension against tether 82 and anchor 83.

FIGS. 10 and 11 show a modification of the embodiment of FIG. 9. As setout supra, the modification comprises reaction plate 22 intermediatebetween the mooring means such as anchors 85 and a pump assemblycomponent such as cylinder 64. The mass and area of reaction plate 22are selected such that the combination affords inertia sufficient toresist the pumping force on piston rod 62.

Although the accumulator-reverse osmosis assembly in this embodimentcould be deployed as in FIG. 9, or in other ways as will occur to theartisan, it is preferred, for reasons of simplicity, to place it on thereaction plate where, although not critical, its inertia contributessomewhat to the forces resisting the rise of the plate 22 from the seafloor. The plate is loosely tethered as shown to anchors 85.

The pump assembly comprising components 10, 20, 62, 64, and 65 not shownjoins the reaction plate 22 via tether 71 and high pressure line 84.

Although the thrust of this disclosure is directed to the desalinationof seawater, it is understood that the device can be applied todesalination in other bodies of water such as brackish lakes oraquifers.

That which is claimed is:
 1. A wave-powered desalination devicecomprising:(a) a wave powered pumping device useful under trade windconditions for producing water pressures of about 800 psi, said devicecomprising:(i) a non-Archimedean wave follower connected in sequencewith; (ii) a piston pump having a cylinder within which is mounted areciprocatory piston; (iii) attachment means connecting one of the pairconsisting of said cylinder and said reciprocatory piston with said wavefollower and the other of said pair with mooring means, alone or incombination with a reaction unit;said pump being in pressurized watercommunication with; (b) a reverse osmosis desalination module, wherebyto desalinate said water.
 2. A wave-powered apparatus for waterdesalination according to claim 1 wherein said pressurized watercommunication comprises a pressure accumulator disposed between thedischarge side of said pump and the inlet side of said reverse osmosismodule.
 3. A wave-powered apparatus for water desalination according toclaim incorporating means returning excess water discharged underpressure from said reverse osmosis unit in coordinated time sequence tothe underside of said reciprocatory piston during the delivery stroke ofsaid pump.
 4. A wave-powered water desalination apparatus according toclaim 1 wherein said buoyant wave follower and said reaction unit,together with the remaining system components, are slack-moored to theocean bottom.